Typical LED systems include an application specific number and configuration of LEDs, a regulator to provide a controlled current to the LED load, and a controller to control the regulator. Some LED applications require the use of multiple parallel strings of series-coupled LEDs and others require only a single string of series-coupled LEDs. Generally controlling the LED current is simpler in the case of a single LED string since the regulator can provide a controlled current to the LED string based on a simple feedback arrangement, such as sensing the voltage across a sense resistor coupled in series with the load.
In some applications for single string LED loads, a bypass or shunt switch may be used to selectively switch in and out a portion of the LEDs in order to achieve different illumination levels. One such application is in automotive head lamps in which part of the LED string can be selectively switched in and out in order to toggle between high beam operation and low beam operation of the head lamps.
Some LED applications require the ability to dim the LEDs. With one type of LED dimming, sometimes referred to as PWM dimming, the intensity of the LEDs is adjusted by turning the LEDs off and on in response to a PWM signal at a variable duty cycle proportional to the desired brightness and with a fixed DC current and frequency (typically 100 Hz to 1 KHz). The PWM signal may be provided to the LED driver from an external source or may be internally generated.